This work examines how the design, features, and moderation policies of online platforms impact user behavior in myriad ways and have significant externalities on society at large. The first two studies examine the effectiveness of different content moderation policies adopted by user-generated content platforms to address issues related to misinformation and verbal aggression, respectively. The third study examines how the design of financial incentive structures affects the behaviors of users on a crowdsourcing platform. The studies produce theoretical implications regarding human behavior on online platforms, from the spreading of misinformation to interpersonal verbal aggression, to the behavioral response to monetary rewards. I additionally make recommendations for practitioners regarding optimal platform design and policies.